Page 10 - PDF_Flip_Book

Chapter 3 - Engines: Knowledge of Engines Is Power

3-27
During a descent, your job is to maintain stable
cylinder head temperatures (CHT) and oil temper-
atures (i.e., keep their temperature indications in
the green). On some airplanes, gear extension or
even partial flap extension at high speeds can be
used in lieu of large power reductions to start a
descent (check your POH). While momentary power
reductions aren’t as harmful if the power is
immediately restored, large ones over long
periods can be damaging. Try planning your
descents so engine temperatures change
slowly from their previous cruise values.

The Propeller
Propellers come in all sizes and colors, but they
are of two basic types: fixed pitch and constant
speed. In an airplane with a fixed pitch prop, one
lever—the throttle—controls both power and pro-
Fig. 46
peller blade RPM (revolutions per minute). In a
constant speed prop, there are separate con-
trols for power and RPM.
When you start your flight training, you’ll
probably fly an airplane with a fixed pitch
propeller. Fixed pitch propellers have their
pitch (angle of attack) fixed during the forg-
ing process. The angle is set in stone (actu-
ally, aluminum). This pitch can’t be changed
except by replacing the propeller, which pret-
ty much prevents you from changing the pro-
peller’s pitch in flight. Fixed pitch props are not
ideal for any one thing, yet they’re in many ways
best for everything. They represent a compromise
between the best
angle of attack for climb and the best angle for cruise. They are simple to oper-
ate, and easier (thus less expensive) to maintain.
On fixed pitch propeller airplanes, engine power and engine RPM are both con-
trolled by the throttle. One lever does it all, power equals RPM, and that’s the end.
As you move up into higher performance airplanes, you’ll soon encounter
constant speed (controllable pitch) propellers. Airplanes with these propellers
usually have both a throttle and a propeller control, so you manage engine power
and propeller RPM separately (Figure 45).
On airplanes with constant speed propellers, movement of the throttle determines
the amount of fuel and air reaching the cylinders. Simply stated, the throttle deter-
mines how much power the engine can develop. Movement of the propeller control
changes the propeller’s pitch (its angle of attack). This directly controls how
fast the propeller rotates (its speed or RPM) as shown in Figure 46. While
throttle determines engine power, propeller pitch determines how efficiently
that power is used. Let’s examine how the controllable propeller works. Then
we’ll examine why changing the propeller’s pitch is useful.
Forward movement of the propeller control causes both halves of the
propeller to rotate about their axes and attack the wind at a smaller angle
(i.e., take a smaller bite of air) as shown in Figure 46A. From aerodynamics,
you know that a smaller angle of attack means less drag and less resistance to
Fig. 45 forward motion. Therefore, moving the propeller control forward increases propeller